Synthesis and mesomorphic behavior of poly(dipropylsilylenemethylene)

Based on the reaction of trichloro(chloromethyl)silane ( 1 ) with 2 equivalent amounts of the respective Grignard-reagent and subsequent cyclization, 1,1,3,3-tetrapropyldisilacyclobutane ( 3 ) has been prepared. Catalytic polymerization with H₂PtCl₆ was employed to prepare the corresponding poly(dipropylsilylenemethylene) (PDPSM, 4 ) with strictly alternating SiR₂/CH₂ backbone structure. A high-molecular-weight fraction of the material (weight-average molecular weight M?_w = 166 500 and number-average molecular weight M?_n = 115 200) obtained by fractionating precipitation was investigated with respect to glass transition and formation of conformationally disordered mesomorphic phases. The glass transition temperature T_g = 232 K of PDPSM evidenced lower backbone flexibility than observed for the analogous poly(dipropylsiloxane), (PDPS). PDPSM exhibited mesomorphic behavior. In contrast to poly(dipropylsiloxane), PDPSM showed a surprisingly narrow mesomorphic regime between 355 K and 365 K. Based on polarizing microscopy and ²⁹Si-MAS (magic angle spinning) solid-state NMR the mesophase is described as a conformationally disordered state, which is most probably columnar in analogy to PDPS.